1. Field of the Invention
The present invention relates to the technical field of a mask to be used in performing lithography to produce devices such as semiconductors.
2. Related Background Art
A method of manufacturing various kinds of products by partly changing the quality of a surface portion of material to be worked through lithography techniques has been widely utilized industrially, especially in the electronic industry. In accordance with this method, products having surface alteration portions of the same pattern can be manufactured in large quantities. Alteration of a surface portion of material to be worked is performed by irradiating the surface portion with various kinds of energy beam radiation. In order to form a pattern, a mask having an energy intercepting member partly disposed, is used. When the irradiated energy beam radiation is visible light or ultraviolet light, a mask is used in which a black opaque pattern made of silver, chrome or the like is provided on a transparent substrate made of glass, quartz or the like.
However, as the formation of a finer pattern is demanded, vacuum ultraviolet rays and X-rays, which are of shorter wavelength, have attracted attention. Such energy beam radiation cannot be transmitted through glass or quartz plate, such as those used as mask substrate members when using visible or ultraviolet light. Thus, glass and quartz are not suitable materials for a mask substrate when using vacuum ultraviolet rays or X-rays.
Thus, in lithography when using vacuum ultraviolet rays or X-rays, a mask is formed by using various kinds of inorganic films, for example, a ceramic film made of silicon, silicon nitride, silicon carbide, diamond or the like, or organic high-polymer thin films made of polyimide, polyamide, polyester or the like, or multi-layered thin films of such films, as an energy beam radiation transmitting element, and by then forming metal such as gold, platinum, tungsten and tantalum on the surfaces of these films, as an energy beam radiation absorbing element.
Among candidates for materials for a mask membrane, Si, BN, SiN and SiC are inorganic materials, which are appropriate for practical use. Among them, much attention has recently been focused on SiC.
Among ceramic materials, SiC is most chemically stable and is superior in mechanical strength. Moreover, the coefficient of thermal expansion of SiC is close to that of Si. Also, SiC is superior in thermal conductivity.
The typical process for forming a SiC membrane film is a CVD (Chemical Vapor Deposition) process. When the formed film, however, is examined, there are various problems. Despite efforts to improve the film by devising production methods and taking additional measures, a solution to basic problems has not been found.
Problems with the conventional SiC membrane films are enumerated below:
(1) The conventional SiC membrane films have poor surface conditions (due to abnormal crystal growth). PA1 (2) The conventional SiC membrane films have low alignment-light transmittance (owing to the inherent absorption and to the scattering in the films and on the surfaces thereof). PA1 (3) Each of the conventional SiC membrane films has nonuniform thickness (it is difficult to control the thickness in each film, in each lot and among lots).
Japanese Patent Appln. Publication Nos. 4-332115 and 7-12017 Official Gazettes have proposed a SiC/Si.sub.3 N.sub.4 film as a countermeasure against these problems. However, a single SiC/Si.sub.3 N.sub.4 film has compressive stress which causes deflection and is thus unsuitable for a membrane for X-ray lithography. It is, therefore, necessary to perform an annealing process or the like for changing the compressive stress into a tensile stress. Further, Japanese Patent Appln. Laid-Open No. 56-140629 Official Gazette also has proposed a SiC/Si.sub.3 N.sub.4 film. However, in this case, two kinds of targets are needed as sputtering targets to laminate a Si.sub.3 N.sub.4 film on a SiC film. Thus, what is called a dual sputtering system has to be prepared as an apparatus for sputtering. Alternatively, it is necessary to perform a complex operation including the steps of breaking a vacuum once and replacing a target with another target.